Pelletization of polyolefin polymer, for example, polybutene-1 copolymer, especially high melt flow and high ethylene (ethylene content higher than 0.75 wt %) butene-1-ethylene copolymer, from molten state has been very difficult when using either the underwater face cutter or the strand cutting pelletizer. The problem appears to relate to the slow crystallization rate or solidification rate of these polymers which exhibit extreme tackiness in pelletizing. It is known that as the pellets leave the cutting blades, they are very clear and tacky, and collide with other pellets to form agglomerates. With a longer residence time, the pellets change to an opaque color, as they complete their crystallization, become hard and lose their tackiness. These agglomerates frequently plug the pelletizer bowl and the spin dryer. The extrusion line has to be shut down in order to clean the plugged section resulting in undesirable production interruptions. Also, the high number of pellet marriages shown in the final product is not acceptable in the customers' fabrication process.
Various attempts have been made to solve or to minimize this problem of undesired pellets agglomeration. Using polymeric nucleants, including polyolefinic nucleants such as polyethylenes (e.g. HDPE powder such as MICROTHENE.RTM.) as an external pelletizing aid to eliminate the surface tackiness is known in the art. Polyolefinic powder nucleants typically have density less than water and thus often float on the surface of the water-bath or tank which in turn results in processing disruptions requiring the occasional cleaning up of equipment and work environment. The success of this method also depends on good dispersion and suspension of HDPE in the slurry water. Therefore, it generally requires vigorous stirring, e.g. using portable mixers, to obtain effective benefit of the HDPE as a pelletizing aid. This method of reducing surface tackiness is very messy and labor intensive. Even under the best of circumstances, a significant fraction (about 20%) of the production has to be discarded due to poor pellet quality and pellet agglomerations.
The use of organometallic surfactants to minimize the pellet agglomeration problem is also known in the art. However, when these surfactants are used at the effective level, it often results in severe foaming problems, and the foam may overflows from the water tank to the floor and thereby causing messy operation and unsafe environment through the obstruction of the view. The combination of a defoamer such as FOAMTROL.RTM. and an organometallic surfactant such as zinc stearate has been used with some success. This comination has the drawback of the potential carcinogenic nature of FOAMTROL.RTM.. FOAMTROL.RTM. is available from Betz Industrial, 4636 Somerton Road, Trevose, Pa. 19047, and the chemical formulation of which is proprietary information of Betz Industrial.
U.S. Pat. No. 4,359,544, assigned to Shell, proposes the use of a nucleating agent package in the polymer melt as pelletizing aid, which package is a mixture of HDPE and stearamide. However, both HDPE and stearamide do not disperse well in water, and thus do not function well in a water bath as pelletizing aids.
U.S. Pat. No. 5,206,292, assgned to Shell, proposes the use of oxidized high density polyethylene powder to minimize the agglomeration of freshly manufactured polyolefin pellets. However, oxidized HDPE is not approved by FDA for applications involving direct food contact.
Thus, there continues to exist the need for new and better methods and/or agents for preventing or minimizing pellet agglomeration in underwater pelletizing operations of polyolefins, especially high melt flow polyolefins.
It has now been discovered that a combination two specific additives cooperate in a synergistic fashion to substantially reduce agglomeration of pellets while dispersing effectively with minimum foaming.